Functional characteristics of mechanoreceptors in sinus hair follicles of the cat

1. The discharge of impulses in afferent fibres dissected from the infraorbital and ulnar nerves of anaesthetized cats was recorded during controlled movements of the maxillary and carpal sinus hairs. 2. Four main types of afferent units were identified. Two had slowly adapting responses characteristic of the epidermal type I, and dermal type II mechanoreceptors of the hairy skin. Two rapidly adapting responses to movement of the sinus hairs were found, one with a high velocity threshold and another with a low velocity threshold. 3. The slowly adapting units showed a power relationship between the degree of displacement of the hair and the mean interspike interval of the response. Slowly adapting units also exhibited a power relationship between the velocity of displacement of a hair and the mean interspike interval of the response. 4. The conduction velocities of all types of afferent units were measured and fell in the range of the Aα, fast myelinated fibres. 5. Movements of the carpal sinus hairs yielded both types of slowly adapting response recorded in fibres of the ulnar nerve directly innervating the carpal sinus hair follicles, and rapidly adapting responses from Pacinian corpuscles, found in close association with, but external to, these follicles. 6. On the basis of the findings in this study and the results of anatomical investigations of the receptor structures in the sinus hair follicle a correlation between the distinguishable afferent responses and the morphologically identifiable nerve endings has been proposed.